富星团中奥尔特云的毁灭
The Destruction of an Oort Cloud in a rich stellar cluster
摘要：
奥尔特云的形成可能可以追溯到太阳系历史的最早时期。在那个时候，太阳几乎可以肯定是它诞生的星团的一员。由于太阳诞生星团可能是巨大的（1000- 10000 Msol），因此寿命很长，一个问题是这样一个原始的奥尔特云的生存。我们通过模拟奥尔特云彗星几百万年的轨道演化来研究这个问题，假设太阳是作为这样一个大质量星团的典型成员开始它的生命的。我们已经设计了一个合成的代表性的相关动态，其中集群的潜力是由国王模型表示，并选择了约20个近距离接触与个别集群恒星和集成的太阳轨道和集群结构的基础上。数千个单独的模拟，每个包括3 000彗星的轨道与三个不同的初始半长轴。实际上，我们发现，对于我们选择的半长轴（5 000- 20 000 au），整个初始的奥尔特云都消失了，与星团质量无关，尽管较小的星团生存的机会更好，因为在一定比例的模拟中，太阳的轨道距离密集的星团中心相对安全。对于我们研究的出生星团大小范围，原始奥尔特云可能仅作为半长轴< 3 000 au的小内核存活。这样的彗星群对轨道扩散到外晕和随后注入可观测轨道是惰性的。因此，如果奥尔特云是原始的，而诞生星团的质量不低，就需要某种机制来完成这种转移。从这个角度来看，我们的结果为奥尔特云的延迟形成提供了一些支持，这发生在太阳离开其诞生星团之后。
英文摘要：
It is possible that the formation of the Oort Cloud dates back to the earliest epochs of solar system history. At that time, the Sun was almost certainly a member of the stellar cluster, where it was born. Since the solar birth cluster is likely to have been massive (1000--10 000 Msol), and therefore long-lived, an issue concerns the survival of such a primordial Oort Cloud. We have investigated this issue by simulating the orbital evolution of Oort Cloud comets for several hundred Myr, assuming the Sun to start its life as a typical member of such a massive cluster. We have devised a synthetic representation of the relevant dynamics, where the cluster potential is represented by a King model, and about 20 close encounters with individual cluster stars are selected and integrated based on the solar orbit and the cluster structure. Thousands of individual simulations are made, each including 3 000 comets with orbits with three different initial semi-major axes. Practically the entire initial Oort Cloud is found to be lost for our choice of semi-major axes (5 000--20 000 au), independent of the cluster mass, although the chance of survival is better for the smaller cluster, since in a certain fraction of the simulations the Sun orbits at relatively safe distances from the dense cluster centre. For the range of birth cluster sizes that we investigate, a primordial Oort Cloud will likely survive only as a small inner core with semi-major axes < 3 000 au. Such a population of comets would be inert to orbital diffusion into an outer halo and subsequent injection into observable orbits. Some mechanism is therefore needed to accomplish this transfer, in case the Oort Cloud is primordial and the birth cluster did not have a low mass. From this point of view, our results lend some support to a delayed formation of the Oort Cloud, that occurred after the Sun had left its birth cluster.
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